Charge control apparatus for controlling charging of an energy storage device via purality of charging paths connected in parallel anssociated energy storage appartus, and an associated charging method

ABSTRACT

This charging control device 50 controls charging of electricity storage elements B1-B4 and is provided with: a plurality of charging paths 61A, 61B leading to the electricity storage elements and connected in parallel with each other; voltage drop elements 64A, 64B and switches 65A, 65B which are connected in series on the charging paths; and a control unit 100. The control unit 100 controls the switches 65A, 65B and thereby switches a charging path not to be energized among the plurality of charging paths 61A, 61B during charging.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application, filed under 35 U.S.C.§ 371, of International Application No. PCT/JP2018/044496, filed Dec. 4,2020, which claims priority to Japan Application No. 2017-232723, filedDec. 4, 2017, the contents of both of which as are hereby incorporatedby reference in their entirety.

BACKGROUND Technical Field

The present invention relates to a technique for charging an energystorage device.

Description of Related Art

As a vehicle engine starting battery, a lithium ion secondary battery(hereinafter, LIB) is installed instead of a lead-acid battery. Themerit of this is that the life of the battery is extended and theacceptance of regenerative charging is improved. Due to the differencein characteristics between the lead-acid battery and the LIB, the chargeset voltage differs, so it is necessary to use a dedicated charger foreach. In consideration of compatibility with lead-acid batteries, theouter shapes and terminal structures have been standardized. However, ifthe LIB is compatible with the lead-acid battery, charging may beperformed at a charge voltage different from the assumed charge setvoltage, such as charging with a charger for the lead-acid battery.Patent Document JP-A-2008-199717 describes that a diode is provided onthe charging path in order to limit charging to the battery.

BRIEF SUMMARY

A large lithium ion secondary battery such as a vehicle engine startingbattery has a large charge/discharge current and a large power loss onthe energization path. Therefore, a method is conceivable in which aplurality of voltage drop elements are connected in parallel to reducethe current flowing through one voltage drop element so as to preventfailure in the voltage drop element such as a diode. However, due tovariations in the characteristics of the elements themselves and thetemperature characteristics, even if a plurality of voltage dropelements are connected in parallel, current may concentrate in one ofthe voltage drop elements, causing a failure. The present invention hasbeen completed based on the above circumstances, and has an object tosuppress failure in a voltage drop element during charging due tocurrent concentration.

A charge control apparatus controls charging of an energy storagedevice. The charge control apparatus includes a plurality of chargingpaths connected in parallel for the energy storage device, a voltagedrop element and a switch connected in series on each of the chargingpaths, and a control unit. The control unit controls the switches toswitch the charging path, of the plurality of charging paths, which isdeenergized during charging.

These techniques can be applied to a method of charging an energystorage device. The techniques can be applied to an energy storageapparatus including an energy storage device and a charge controlapparatus. The techniques can be implemented in various forms such as anenergy storage system, a charging path switching program, and arecording medium recording the program.

With this configuration, it is possible to suppress failure in a voltagedrop element during charging due to current concentration.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a side view of an automobile according to the firstembodiment.

FIG. 2 is a perspective view of a battery.

FIG. 3 is an exploded perspective view of the battery.

FIG. 4 is a block diagram showing the electrical configuration of thebattery.

FIG. 5 is a chart showing the ON/OFF switching of each FET.

FIG. 6 is a diagram showing the switching between charging paths.

FIG. 7 is a diagram showing the switching between the charging paths.

FIG. 8 is a flowchart for the switching between the charging paths.

FIG. 9 is a graph showing the transition of the total voltage of anassembled battery during charging.

FIG. 10 is a diagram showing a comparative example of a charge circuit.

FIG. 11 is a graph showing the temperature characteristics of theforward voltage of a diode.

FIG. 12 is a flowchart for the switching between the charging pathsaccording to the second embodiment.

FIG. 13 is a block diagram showing another embodiment of the chargingpaths.

FIG. 14 is a block diagram showing another embodiment of the chargingpaths.

FIG. 15 is a chart showing the switching between the charging paths.

FIG. 16 is a block diagram showing another embodiment of the chargingpaths.

FIG. 17 is a chart showing the ON/OFF switching of each FET.

FIG. 18 is a chart showing the ON/OFF switching of each FET.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

A charge control apparatus controls charging of an energy storagedevice. The charge control apparatus includes a plurality of chargingpaths connected in parallel for the energy storage device, a voltagedrop element and a switch connected in series on each of the chargingpaths, and a control unit. The control unit controls the switches toswitch the charging path, of the plurality of charging paths, which isdeenergized during charging.

This configuration enables to lower the charge voltage for the energystorage device by the voltage drop element arranged in the chargingpath. Moreover, among the plurality of charging paths, the charging paththat is not energized is switched during charging, so that it ispossible to prevent current from concentrating on some of the chargingpaths and the temperature of the voltage drop element from increasing.This can suppress failure in the voltage drop element and safely chargethe energy storage device.

The voltage drop element may be a diode. The diode has a temperaturecharacteristic that the forward voltage decreases as the temperatureincreases. Therefore, current is likely to concentrate on one diodewhose temperature has risen, making parallel connection difficult.Applying this technology can prevent current from concentrating on onediode and hence can suppress failure in the diode.

The control unit may switch between the charging paths when the chargevoltage or the voltage of the energy storage device is higher than a setvoltage.

This configuration enables to lower the charge voltage when the chargevoltage or the voltage of the energy storage device is higher than theset voltage. Therefore, the energy storage device can be charged safely.

The control unit may switch between the charging paths based on a settime. With this configuration, because the charging paths are switchedbased on the set time, it is possible to prevent current from flowingunevenly in a specific charging path.

The control unit may switch between the charging paths based on atemperature condition for the charging paths. In this configuration, thecharging path are switched according to a temperature condition for thevoltage drop element. Therefore, it is possible to suppress abnormalheat generation of the voltage drop element regardless of theenvironmental temperature and the situation in the energy storageapparatus.

The control unit may control the switches in the charging paths suchthat during the switching control of the charging paths, the switches inthe charging paths have an overlapping period in which the switches aresimultaneously turned on. This configuration enables to prevent theplurality of switches from being turned off at the same time when thecharging paths are switched, thereby preventing the stoppage ofcharging.

At least one charging path among the charging paths connected inparallel may have two FETs that are back-to-back connected and havebuilt-in parasitic diodes. In this configuration, of the twoback-to-back connected FETs, the FET on one side, whose built-inparasitic diode is in the forward direction with respect to the chargingdirection, is turned off, whereas the FET on the other side, whosebuilt-in parasitic diode is in the reverse direction with respect to thecharging direction, is turned on, thereby causing charge current to flowinto the energy storage device via the FET and the parasitic diode onthe other side. This makes it possible to use the parasitic diode as avoltage drop element. Back-to-back connection means connecting two FETsback to back, that is, connecting the drains of the two FETs orconnecting the sources of the two FETs. An FET is a field effecttransistor.

Each of the charging paths connected in parallel has two FETsback-to-back connected and having built-in parasitic diodes. The controlunit may perform control to simultaneously turn on the two back-to-backconnected FETs in at least one of the charging paths connected inparallel when the charge voltage or the voltage of the energy storagedevice is lower than a set voltage and, when the charge voltage or thevoltage of the energy storage device is lower than a set voltage, mayperform controls to turn off the FETs on one side, of the two FETsinstalled in each of the charging paths and back-to-back connected, inwhich the built-in parasitic diodes are in a forward direction withrespect to a charging direction and turn on the FETs on the other sidein which the built-in parasitic diodes are in a reverse direction withrespect to the charging direction at different timings between thecharging paths. When the timings to turn on differ, it means that the ONdurations do not match exactly. This includes a case in which the FETsare turned on alternately such that the ON durations do not overlap anda case in which the switches to be turned on are switched while the ONdurations partly overlap. In short, with regard to the FET on the otherside, whose built-in parasitic diode is in the reverse direction withrespect to the charging direction, in order to allow charging, thecontrol unit performs control to turn on at least one FET in eachcharging path so as to prevent all the FETs from being turned off.Further, the FETs may be controlled such that the ON durations differfrom each other between the respective charging paths.

In this configuration, in each charging path, two back-to-back FETs areturned on at the same time, so that charge current can flow withoutpassing through the parasitic diodes. Therefore, it is not necessary toprovide a dedicated charging path for normal operation, and the circuitconfiguration is simple. Further, when the charge voltage or the voltageof the energy storage device is higher than the set voltage, the chargecurrent is made to flow via the parasitic diode of the FET while thecharging paths are switched. This makes it possible to lower the chargevoltage of the energy storage device while suppressing the temperaturerise of the parasitic diode.

First Embodiment

1. Description of Battery

FIG. 1 is a side view of an automobile. FIG. 2 is a perspective view ofa battery. FIG. 3 is an exploded perspective view of the battery. FIG. 4is a block diagram showing the electric configuration of the battery.

As shown in FIG. 1 , an automobile 1 includes a battery 20 that is anenergy storage apparatus. As shown in FIG. 2 , the battery 20 has ablock-shaped battery case 21. The battery case 21 accommodates anassembled battery 30 including a plurality of secondary batteries B1 toB4 and a control board 28.

As shown in FIG. 3 , the battery case 21 includes a box-shaped case mainbody 23 that opens upward, a positioning member 24 that positions theplurality of secondary batteries B1 to B4, and an inner lid 25 and anupper lid 26 which are mounted on the upper part of the case main body23. In the case main body 23, as shown in FIG. 3 , a plurality of cellchambers 23A for individually accommodating the respective secondarybatteries B1 to B4 are provided side by side in the X direction.

As shown in FIG. 3 , the positioning member 24 has a plurality of busbars 27 disposed on the upper surface. The positioning member 24 isdisposed on the upper part of the plurality of secondary batteries B1 toB4 disposed in the case main body 23 so as to position the plurality ofsecondary batteries B1 to B4 and connect them in series via theplurality of bus bars 27.

The inner lid 25 has a substantially rectangular shape in plan view, asshown in FIG. 2 . At both end portions of the inner lid 25 in the Xdirection, a pair of terminal portions 22P and 22N to which a harnessterminal (not shown) is connected are provided. The pair of terminalportions 22P and 22N are made of, for example, a metal such as a leadalloy. The terminal portion 22P is a positive electrode terminal portionand the terminal portion 22N is a negative electrode terminal portion.

On the upper surface of the inner lid 25, an accommodation portion 25Ais provided. The control board 28 is accommodated inside theaccommodation portion 25A of the inner lid 25. When the inner lid 25 isattached to the case main body 23, a secondary battery B and the controlboard 28 are connected. Further, the upper lid 26 is mounted on theupper part of the inner lid 25 so as to close the upper surface of theaccommodation portion 25A accommodating the control board 28.

The electrical configuration of the battery 20 will be described withreference to FIG. 4 . The battery 20 is a 12-V system for starting theengine and includes the assembled battery 30, a current sensor 41, avoltage detector 45, and a charge control apparatus 50.

The assembled battery 30 includes four lithium ion secondary batteriesB1 to B4 connected in series. The lithium ion secondary batteries B1 toB4 are an example of the “energy storage device” according to thepresent invention.

The current sensor 41 is provided inside the battery case 21 and detectsa current I flowing through the assembled battery 30. The current sensor41 is electrically connected to a management unit 100 via a signal line,and an output from the current sensor 41 is captured by the managementunit 100.

The voltage detector 45 is provided inside the battery case 21 anddetects battery voltages V1 to V4 of the respective lithium ionsecondary batteries B1 to B4 and a total voltage Ev of the assembledbattery 30. The voltage detector 45 is electrically connected to amanagement unit 100 via a signal line, and an output from the voltagedetector 45 is captured by the management unit 100.Ev=V1+V2+V3+V4

The charge control apparatus 50 includes a charge circuit 60 and themanagement unit 100. The charge circuit 60 includes a first chargingpath 61A, a second charging path 61B, and a temperature sensor 67. Thefirst charging path 61A and the second charging path 61B are between thepositive electrode of the assembled battery 30 and the positiveelectrode side terminal portion 22P, and are connected in parallel witheach other.

A first FET 63A and a second FET 65A are provided in the first chargingpath 61A. The first FET 63A and the second FET 65A are P-channel fieldeffect transistors, and are back-to-back connected. Specifically, thefirst FET 63A has a source connected to the positive electrode of theassembled battery 30, and the second FET 65A has a source connected tothe positive electrode side terminal portion 22P. The drains of thefirst FET 63A and the second FET 65A are commonly connected. The firstFET 63A has a built-in parasitic diode 64A, and the second FET 65A has abuilt-in parasitic diode 66A. The forward direction of the parasiticdiode 64A is the same as the charging direction, and the forwarddirection of the parasitic diode 66A is the same as the dischargingdirection.

A first FET 63B and a second FET 65B are provided in the second chargingpath 61B. The first FET 63B and the second FET 65B are P-channel fieldeffect transistors, and are back-to-back connected. Specifically, thefirst FET 63B has a source connected to the positive electrode of theassembled battery 30, and the second FET 65B has a source connected tothe positive electrode side terminal portion 22P. The drains of thefirst FET 63B and the second FET 65B are commonly connected. The firstFET 63B has a parasitic diode 64B, and the second FET 65B has aparasitic diode 66B. The forward direction of the parasitic diode 64B isthe same as the charging direction, and the forward direction of theparasitic diode 66B is the same as the discharging direction.

The temperature sensor 67 detects the temperature of each of the FETs63A, 63B, 65A, and 65B. The temperature sensor 67 is electricallyconnected to a management unit 100 via a signal line, and an output fromthe temperature sensor 67 is captured by the management unit 100.

The management unit 100 includes a CPU (central processing unit) 101having a calculation function, a ROM 103, a memory 105, and acommunication unit 107 and is provided on the control board 28.

The CPU 101 monitors the current I flowing through the assembled battery30 based on an output from the current sensor 41. The CPU 101 monitorsthe voltages V1 to V4 of the respective lithium ion secondary batteriesB1 to B4 and the total voltage Ev of the assembled battery 30 based onoutputs from the voltage detector 45. Further, the CPU 101 monitors thetemperature of each of the FET 63A, 65A, 63B, and 65B based on an outputfrom the temperature sensor 67.

During charging, the CPU 101 detects the magnitude of the total voltageEv of the assembled battery 30 and executes a switching procedure forswitching between the charging paths 61A and 61B for the assembledbattery 30. The CPU 101 corresponds to the “control unit” in the presentinvention.

The ROM 103 stores a program for executing the charging path switchingprocedure (S10 to S50 shown in FIG. 8 ). The program can be stored in arecording medium such as a CD-ROM and transferred. The program can bedistributed via a telecommunication circuit.

The communication unit 107 is provided for communication with a vehicleECU (Electronic Control Unit) 150 mounted in the automobile 1. Uponbeing mounted in the vehicle, the communication unit 107 is connected tothe vehicle ECU 150 via a signal line. The management unit 100 canreceive information about the vehicle such as the operating state of theengine (stopped or driven) from the vehicle ECU 150.

As shown in FIG. 4 , the battery 20 is connected to a starter motor 160for starting the engine, a vehicle load such as electrical components,and an alternator 170. When the amount of power generated by thealternator 170 is larger than the power consumption of the vehicle loadduring engine driving, the battery 20 is charged by the alternator 170.

When the amount of power generated by the alternator 170 is smaller thanthe power consumption of the vehicle load, the battery 20 is dischargedto make up for the shortage. While the engine is stopped, the alternator160 stops generating power. Therefore, the battery 20 is in a state inwhich power supply is stopped (state in which the battery is notcharged), and is in a state in which the battery is only discharged forthe vehicle load.

The battery 20 can be charged by connecting an external charger 180outside the vehicle, for example, during parking, in addition to thevehicle-mounted alternator 170. Both the alternator 170 and the externalcharger 180 outside the vehicle are DC outputs.

The battery 20 includes the lithium ion secondary batteries B1 to B4,the current sensor 41, the voltage detector 45, and the charge controlapparatus 50, and thus corresponds to the “energy storage apparatus”according to the present invention.

2. Charge Voltage Control and Parallel Connection of Diodes

Due to the difference in characteristics, lead-acid batteries andlithium ion secondary batteries have different charge set voltages (theset values of charge voltages). In the case of the 12 V system, a chargeset voltage Eo for lead-acid batteries is 14.8 V and the charge setvoltage Eo for lithium ion secondary batteries is 14 V.

For example, when the battery 20 that uses a lithium ion secondarybattery as an energy storage device is charged by an external chargerfor lead-acid storage, charging is sometimes performed at a voltage(14.8 V) higher than the assumed charge set voltage (14 V). For safetyreasons, it is preferable that the charge voltage does not exceed thecharge set voltage Eo.

Therefore, when the total voltage Ev of the assembled battery 30 exceedsthe assumed charge set voltage Eo, the charge voltage may be lowered byusing the voltage drop effect of a diode D provided in a charging path Las shown in FIG. 10 . In order to prevent the diode D from generatingheat and failing due to power loss on the charging path, it is possibleto connect a plurality of diodes D in parallel to divide the current.Referring to FIG. 10 , three diodes D1 to D3 are connected in parallel.

As shown in FIG. 11 , the diode D has a characteristic that a forwardvoltage Vf decreases as the temperature increases.

Since the diodes D have individual differences and variations indischarge performance, the diodes D1 to D3 connected in parallel havevariations in temperature rise during charging. In the diode D thatincreases in temperature more than the other diodes, the forward voltageVf decreases more to allow current to easily flow. Therefore, even ifthe plurality of diodes D1 to D3 are connected in parallel, currentconcentrates on some of the diodes D whose temperatures have risen, andthe diodes D fail.

Accordingly, when the total voltage Ev of the assembled battery 30 ishigher than the charge set voltage Eo during charging, the CPU 101performs control to turn off the FET 63A in the first charging path 61Aand the FET 63B in the second charging path 61B as shown in FIG. 5 .Further, the CPU 101 inputs an ON signal (a signal for controlling toturn on the FET) to the FET 65A in the first charging path 61A at apredetermined control cycle Ts, while inputting an ON signal to the FET65B in the second charging path 61B at a cycle shifted by half thecycle. Inputting ON signals to the respective FETs 65A and 65B will turnon them alternately for each half cycle of the control cycle Ts.

As a result, the two charging paths 61A and 61B are switched between theenergized state and the deenergized state every half cycle Ts/2 of thecontrol cycle Ts. As shown in FIG. 6 , the charge current to theassembled battery 30 alternately flows in the charging paths 61A and 61Bsuch that when the charge current flows in one charging path (forexample, 61A), the other charging path (for example, 61B) isdeenergized. The half cycle Ts/2 corresponds to the “set time” in thepresent invention.

This causes a voltage drop due to the parasitic diode 64A or 64Bregardless of in which of the first charging path 61A and the secondcharging path 61B the charge current is flowing. Therefore, the chargevoltage for the assembled battery 30 can be lowered.

Because the charge current is made to alternately flow through the twocharging paths 61A and 61B, current does not concentrate on one of theparasitic diodes 64A and 64B. This can prevent failure in the parasiticdiodes 64A and 64B.

FIG. 8 shows a charging path switching procedure executed by the CPU101. The switching procedure is composed of five steps S10 to S50, andis executed when the CPU 101 detects the charging of the assembledbattery 30. Whether or not the battery is charged can be determinedbased on the current detected by the current sensor 41.

A charging path switching operation will be described by taking as anexample the case in which the battery 20 is charged by the externalcharger 180 for lead-acid storage. The charge voltage of the externalcharger 180 is 14.8 V.

When the CPU 101 detects charging by the external charger 180, the CPU101 acquires the total voltage Ev of the assembled battery 30 from anoutput from the voltage detector 45 (S10).

The CPU 101 then compares the total voltage Ev of the assembled battery30 with the charge set voltage Eo, and determines whether the totalvoltage Ev of the assembled battery 30 is equal to or lower than thecharge set voltage Eo (S20). The charge set voltage Eo is 14 V.

FIG. 9 is a graph showing a change in the voltage of the assembledbattery after the start of charging. As shown in FIG. 9 , at time t0immediately after the start of charging, the total voltage Ev of theassembled battery 30 is less than 14 V, and hence YES is obtained instep S20.

If YES is obtained in step S20, the CPU 101 performs control to turn onall the FETs 63A, 65A, 63B, and 65B provided in the first charging path61A and the second charging path 61B. As a result, as shown in FIG. 7 ,charge current from the external charger 180 branches and flows into thetwo charging paths 61A and 61B to charge the assembled battery 30 (S30).Subsequently, the CPU 101 determines whether the charging is completed(S40). If the charging is not completed, the process returns to stepS10.

After the start of charging, the assembled battery 30 is charged by thecharge current that branches and flows into the two charging paths 61Aand 61B, and the total voltage Ev rises. When all the FETs are turnedon, no current flows in parasitic diodes 64 and 66, so that charging isperformed without lowering the charge voltage.

When the total voltage Ev of the assembled battery 30 reaches the chargeset voltage Eo (time t1 in FIG. 9 ), NO is obtained in step S20.

If NO is obtained in S20, the CPU 101 performs control to turn off theFET 63A in the first charging path 61A and the FET 63B in the secondcharging path 61B. The FET 65A in the first charging path 61A and theFET 65B in the second charging path 61B are controlled so as to bealternately turned on and off.

As a result, during the period T1 after the total voltage Ev of theassembled battery 30 reaches the charge set voltage Eo, as shown in FIG.6 , the charge current alternately flows from the external charger 180to the two charging paths 61A and 61B, thus charging the assembledbattery 30 until the charging is completed (S40).

The charging of the assembled battery 30 ends when the charging endcondition is satisfied such that the value of the charge current becomesa predetermined value or less or reaches an upper limit voltage Em.

This causes a voltage drop due to the parasitic diode 64A or 64Bregardless of in which of the first charging path 61A and the secondcharging path 61B the charge current is flowing in a period T1 after thetotal voltage Ev of the assembled battery 30 reaches the charge setvoltage Eo.

Accordingly, when the charge voltage (output voltage) of the externalcharger 180 is higher than the charge set voltage Eo, the charge voltagecan be lowered by the parasitic diodes 64A and 64B to charge theassembled battery 30.

FIG. 9 shows the transition of the total voltage Ev of the assembledbattery 30 at the time of charging. The “solid line” indicates thetransition of the voltage when the control for lowering the chargevoltage is executed, and the “dashed line” indicates the transition ofthe voltage when the control for lowering the charge voltage is notexecuted. The upper limit voltage Em shown in FIG. 9 is the upper limitvoltage (the voltage at which charging is stopped) of the assembledbattery 30. The upper limit voltage Em is a value higher than the chargeset voltage of 14 V and lower than the charge voltage of 14.8 V of theexternal charger 180, and is, for example, 14.5 V.

After the start of charging, at time t1 in FIG. 9 , the total voltage Evof the assembled battery 30 exceeds the charge set voltage Eo. If thecontrol to lower the charge voltage is not executed, the total voltageEv of the assembled battery 30 rises afterward. At time t2 when thetotal voltage Ev reaches the upper limit voltage Em, the protectingoperation of shutting off the charge circuit 60 (turning off all theFETs to cut off current) is effected to stop charging.

When the control for lowering the charge voltage is executed, thecontrol for alternately turning on the two FETs 65A and 65B is executedto lower the charge voltage for the assembled battery Ev from 14.8 V toabout 14.2 V at the timing of time t1 shown in FIG. 9 when the totalvoltage Ev of the assembled battery 30 exceeds the charge set voltageEo. Therefore, it is possible to prevent the total voltage Ev of theassembled battery 30 from rising to the upper limit voltage of 14.5 V,and it is possible to continue charging the assembled battery 30 aftertime t2.

As described above, during charging by the external charger 180, whenthe total voltage Ev of the assembled battery 30 becomes higher than thecharge set voltage Eo, a current is made to flow through the parasiticdiodes 64A and 64B to lower the charge voltage. In addition to this,when the total voltage Ev of the assembled battery 30 becomes higherthan the charge set voltage Eo during charging by the alternator 170, acurrent may be made to flow through the parasitic diodes 64A and 64B tolower the charge voltage. The operation during charging has been mainlydescribed above. However, at the time of discharging, it is preferableto turn on all the FETs 63A, 63B, 65A, and 65B to branch the dischargecurrent to the two charging paths 61A and 61B.

3. Description of Effect

In this configuration, when the total voltage Ev of the assembledbattery 30 is higher than the charge set voltage Eo, the charge voltagefor the assembled battery 30 can be lowered by the charge circuit 60inside the battery. This improves the safety of the battery 20. Inaddition, because the charge current is made to alternately flow throughthe two charging paths 61A and 61B, current does not concentrate on oneof the parasitic diodes 64A and 64B. This can suppress failure in theparasitic diodes 64A and 64B.

In this configuration, because the charge voltage of the externalcharger 180 is 14.8 V and the upper limit voltage of the assembledbattery 30 is 14.5 V, the charge voltage is lowered by 0.6 V by theparasitic diodes 64A and 64B, so that the total voltage Ev of theassembled battery 30 can be suppressed to equal to or less than theupper limit voltage of 14.5 V. The voltage drop amount of the chargevoltage due to a voltage drop element such as a parasitic diode ispreferably such that the charge voltage after the drop is lower than theupper limit voltage Em so as not to make the total voltage Ev of theassembled battery 30 exceed the upper limit voltage Em. Even if thecharge voltage after the drop is higher than the upper limit voltage,the period until the total voltage Ev of the assembled battery 30reaches the upper limit voltage Em is delayed compared to the case inwhich the charge voltage is not lowered. This provides the advantage ofprolonging the charging time of the assembled battery 30 accordingly.

In this configuration, the two charging paths 61A and 61B are switchedfor each half cycle Ts/2, that is, for each set time, so that the chargecurrent can be prevented from being disproportionately flowing throughthe specific charging path 61A or 61B. This can suppress failure in theparasitic diodes 64A and 64B.

The lithium ion secondary batteries B1 to B4 have higher internalresistance at low temperature than other secondary batteries, and hencetend to reach an overvoltage condition when charged at low temperature.As a countermeasure against this problem, it may be possible to suppressthe charge current. In this configuration, when the total voltage Ev ofthe assembled battery 30 is higher than the charge set voltage Eo, thecharge voltage is controlled to be lowered. Because the charge currentcan be suppressed by lowering the charge voltage, it is possible toprevent the lithium ion secondary batteries B1 to B4 from reaching anovervoltage condition due to charging at low temperature.

Second Embodiment

In the first embodiment, when the total voltage Ev of the assembledbattery 30 is higher than the charge set voltage Eo, the CPU 101performs control to alternately switch between the two charging paths61A and 61B between the energized state and the deenergized state byalternately inputting an ON signal (a signal for controlling to turn onthe FET) to the FET 65A in the first charging path 61A and the FET 65Bin the second charging path 61B while shifting the signal by the halfcycle Ts/2.

In the second embodiment, when a total voltage Ev of an assembledbattery 30 is higher than a charge set voltage Eo, a CPU 101 performscontrol to switch between charging paths 61A and 61B according totemperature conditions for an FET 63A and an FET 63B, specifically, atemperature difference. The temperature information of the FETs 63A and63B can be acquired by a temperature sensor 67.

FIG. 12 is a flowchart for charging path switching control based on atemperature difference. When the total voltage Ev of the assembledbattery 30 is higher than the charge set voltage Eo, the CPU 101performs control to turn off the FET 63A in a first charging path 61Aand the FET 63B in a second charging path 61B.

In addition, the CPU 101 performs control to turn off an FET 65A in thefirst charging path 61A and an FET 65B in the second charging path 61B.As a result, only the first charging path 61A is set in the energizedstate, and the assembled battery 30 is charged through the firstcharging path 61A (S100).

The CPU 101 acquires a temperature Ta of the FET 63A in the firstcharging path 61A and a temperature Tb of the FET 63B in the secondcharging path 61B from outputs from the temperature sensor 67 whilecharging is performed using the first charging path 61A.

The CPU 101 then calculates a temperature difference Ta−Tb and performsthe processing of comparing with a threshold Th. When the temperaturedifference Ta−Tb is smaller than the threshold Th, the CPU 101 continuescharging via the first charging path 61A (S110: YES).

On the other hand, when the temperature difference Ta−Tb is larger thanthe threshold Th, the CPU 101 switches the charging path from the firstcharging path 61A to the second charging path 61B and performs charging(S120). Specifically, the CPU 101 switches the charging path byswitching the FET 65A in the first charging path 61A from on to off andswitching the FET 65B in the second charging path 61B from off to on.

After switching between the charging path, the CPU 101 calculates atemperature difference Tb−Ta from an output from the temperature sensor67 and compares it with the threshold Th.

When the temperature difference Tb−Ta is smaller than the threshold Th,the CPU 101 continues charging via the second charging path 61B (S130:YES).

On the other hand, when the temperature difference Tb−Ta is larger thanthe threshold Th, the CPU 101 switches the charging path from the firstcharging path 61A to the second charging path 61B and performs charging(S100).

As described above, in the second embodiment, when the temperaturedifferences Ta−Tb and Tb−Ta between the two FETs 63A and 63B becomelarger than the threshold Th, the charging path is switched. Thetemperature differences Ta−Tb and Tb−Ta between the two FETs 63A and 63Bcan be kept smaller than the threshold Th. It is possible to prevent thetemperature of parasitic diodes 64A and 64B from rising during chargingto cause failure in the FETs 63A and 63B. In this configuration, thecharging paths 61A and 61B are switched according to the actualtemperatures of the FETs 63A and 63B. Therefore, it is possible tosuppress abnormal heat generation of the parasitic diodes 64A and 64B asvoltage drop elements regardless of the environmental temperature andthe situation in the battery.

OTHER EMBODIMENTS

The present invention is not limited to the embodiments described withreference to the above description and the drawings. For example, thefollowing embodiments are also included in the technical scope of thepresent invention.

(1) The energy storage device is not limited to the lithium ionsecondary batteries B1 to B4, and may be other types of secondarybatteries. They may be capacitors and the like. Although the first andsecond embodiments exemplify the configuration in which the plurality oflithium ion secondary batteries B1 to B4 are connected in series, aserial/parallel connection or a single cell configuration may be used.

The battery 20 is used for automobiles (four-wheeled vehicles) asdescribed above, but may be used for motorcycles, electric vehicles, andhybrid electric vehicles. In addition, the battery is used to start theengine as described above, but may be used as an auxiliary battery. Theuse of the battery is not limited to a vehicle, but can be applied to,for example, a UPS or the energy storage apparatus of a solar powergeneration system. The battery 20 mounted on a motorcycle may not have acommunication function with the motorcycle. Assume that the battery 20has no communication function. In this case, even if the charge voltage(output voltage) of the alternator 170 mounted on the motorcycle ishigher than the charge set voltage, it is not possible to performadjustment to lower the charge voltage by sending a command from thebattery 20 to the alternator 170. The battery 20 mounted on themotorcycle has a problem that the battery tends to be charged with acharge voltage higher than the charge set voltage. Applying thistechnology to the battery 20 mounted on the motorcycle can lower thecharge voltage for the assembled battery 30 by the charge circuit 60 andprevent the assembled battery 30 from reaching an overvoltage.

(2) In the first embodiment, the CPU 101, which is the control unit,lowers the charge voltage for the assembled battery 30 by performingcontrol to switch between the charging paths 61A and 61B during chargingwith the external charger 180. When the charge voltage (output voltage)of the alternator 170 is higher than the charge set voltage while theautomobile 1 is running, the CPU 101, which is the control unit, maylower the charge voltage for the assembled battery 30 by performingcontrol to switch between the charging paths 61A and 61B.

(3) Even though the charge voltage of the alternator 170 is higher thanthe charge set voltage, if the period during which the charge voltage ishigher than the charge set voltage is shorter than a predetermined time,the CPU 101 need not execute control to lower the charge voltage byswitching between the charging paths 61A and 61B. It is possible tosuppress switching between the charging paths 61A and 61B when thecharge voltage of the alternator 170 temporarily rises due toregenerative charging accompanying deceleration of the automobile 1. Thepredetermined time is, for example, a short time of about 50 msec.

(4) The first embodiment has exemplified the configuration in which thecharge circuit 60 and the management unit 100 are provided inside thebattery 20. The charge circuit 60 and the management unit 100 need notnecessarily be installed inside the battery 20, and may be installedoutside the battery 20 as long as they are mounted on the vehicle. Thatis, the battery 20 may be composed of only the lithium ion secondarybatteries B1 to B4 and sensors for measuring voltage and current, andthe management unit 100 provided outside the battery may monitor outputsfrom the sensors and switch between the charging paths 61A and 61Bprovided outside the battery. That is, this technology can also beapplied to an energy storage system including the energy storageapparatus (the battery 20) including only an assembled battery andsensors, the charge circuit 60 located outside the energy storageapparatus, and the control unit (the management unit 100) locatedoutside the energy storage device. Further, the embodiment hasexemplified the configuration in which the charge circuit 60 is arrangedon the positive electrode side of the assembled battery 30. The chargecircuit (a circuit in which voltage drop elements and switches arearranged on a plurality of charging paths) may be arranged on thenegative electrode side.

(5) In the first and second embodiments, the charge voltage is loweredby using the parasitic diodes 64A and 64B of the FETs 63A and 63B. Thevoltage drop element may be any element that causes a voltage drop whena current flows, and may be other than a diode. In particular, in thecase of elements each of which causes a voltage drop when a currentflows and has a negative temperature coefficient (the higher thetemperature, the smaller the resistance value), if the elements areconnected in parallel, current concentrates on some of the elementswhose temperature has risen. Therefore, this technology may be appliedto a case in which a diode is substituted for an element having anegative temperature coefficient. Since the collector-emitter of thetransistor has a negative temperature coefficient and a saturationvoltage of about 0.3 V (Vce=0.3 V), it can be used in place of thediode.

(6) In the first and second embodiments, the two FETs 63A and 65Aconnected back to back are arranged in the charging path 61A, and thetwo FETs 63B and 65B connected back to back are arranged in the chargingpath 61B. The FET 63A and 63B can be replaced with a diode (singleunit). Further, the FETs 65A and 65B may be switches and can be replacedwith bipolar transistors or the like.

(7) In the first embodiment, the two FETs 63A and 65A connected back toback are arranged in the charging path 61A, and the two FETs 63B and 65Bconnected back to back are arranged in the charging path 61B. Inaddition to this, the two FETs 63A and 65A connected back-to-back arearranged in only one charging path 61A, and a single diode and a singleswitch may be arranged in place of the two FETs 63B and 65B connectedback-to-back in the other charging path 61B. Back-to-back connection maybe performed so as to connect the sources to each other instead ofconnecting the drains of the FETs to each other.

(8) In the first embodiment, when the total voltage Ev of the assembledbattery 30 is equal to or lower than the charge set voltage Eo, all thefour FETs 63A, 63B, 65A, and 65B are turned on to allow energization ofboth the first charging path 61A and the second charging path 61B.However, only the FETs 63A and 65A in the first charging path 61A may beturned to allow energization of only the first charging path 61A.

(9) The first and second embodiments have exemplified the case in whichwhen the total voltage Ev of the assembled battery 30 is higher than thecharge set voltage Eo, a current is made to flow through the parasiticdiodes 64A and 64B of the FETs 63A and 63B to lower the charge voltage.In addition to this, when the charge voltage (output voltage) of thealternator 170 or the external charger 180 is higher than the charge setvoltage Eo, current may be made to flow through the parasitic diodes 64Aand 64B of the FETs 63A and 63B to lower the charge voltage. A chargevoltage may be obtained by detecting the potential difference betweenthe pair of terminal portions 22P and 22N with a sensor.

If it is known in advance that a charger whose charge voltage is higherthan the charge set voltage Eo is used, currents may be made to flowthrough the parasitic diodes 64A and 64B of the FETs 63A and 63B toalways lower the charge voltage during charging regardless of themagnitude relationship between the charge voltage and the charge setvoltage Eo and the magnitude relationship between the total voltage Evand the charge set voltage Eo. Depending on the purpose of use and theenvironment of use, currents may be made to flow through the parasiticdiodes 64A and 64B of the FETs 63A and 63B to always lower the chargevoltage. The present invention can be applied to a case in which it isdesired to delay the deterioration of the assembled battery 30 bylowering the charge voltage or the assembled battery 30 is to be usedwhile being charged with a voltage lower than a setting.

(10) In the first and second embodiments, two paths, that is, the firstcharging path 61A and the second charging path 61B are provided ascharging paths to the assembled battery 30. When the charge voltage isequal to or lower than the charge set voltage Eo, charging is performedby branching a current into the two charging paths 61A and 61B, whereaswhen the charge voltage is higher than the charge set voltage Eo,charging is performed by making a current alternately flow through thetwo charging paths 61A and 61B. In addition to the above, like a battery300 shown in FIG. 13 , a battery may be provided with a charging path(main path) Lo to be used when the charge voltage is equal to or lowerthan the charge set voltage Eo and charging paths (sub-paths for voltagedrop) L1 and L2 to be used when the charge voltage is higher than thecharge set voltage Eo. The charging path Lo is provided with a switchSWo such as a relay and not provided with a voltage drop element such asa diode. The charging path L1 and the charging path L2 are connected inparallel with the charging path Lo, the charging path L1 is providedwith a diode D1 and a switch SW1, and the charging path L2 is providedwith a diode D2 and a switch SW2.

According to the battery 300, when the charge voltage is equal to orlower than the charge set voltage Eo, the management unit 100 turns ononly the switch SWo and turns off the switches SW1 and SW2. As a result,when the charge voltage is equal to or lower than the charge set voltageEo, only the charging path Lo having no voltage drop element isenergized, and a charge current from an external charger 180 flowsthrough the charging path Lo to charge an assembled battery 30.

On the other hand, when the charge voltage is higher than the charge setvoltage Eo, the management unit 100 turns off the switch SWo andalternately turns on the switches SW1 and SW2. As a result, when thecharge voltage is higher than the charge set voltage Eo, the chargingpath Lo is set in the deenergized state, and the charging paths L1 andL2 having the diodes D1 and D2 are alternately set in the energizedstate. Therefore, a charge current from the external charger 180alternately flows through the charging paths L1 and L2 to charge theassembled battery 30. The charge voltage can be lowered by voltage dropdue to the diodes D1 and D2 provided in the charging paths L1 and L2.

(11) The charging paths used when the charge voltage is higher than thecharge set voltage Eo may include two parallel paths or more. As shownin FIG. 14 , four charging paths L1 to L4 may be connected in parallel.To switch between the charging paths L1 to L4, the charging path to bedeenergized may be switched during charging. For example, as shown inFIG. 15 , the plurality of charging paths L1 to L4 to be energized maybe switched in a shifted manner at predetermined time intervals.

(12) A battery 400 shown in FIG. 16 has a configuration provided withtwo charge circuits 460A and 460B. The charge circuit 460A and thecharge circuit 460B differ in the number of series diodes D arranged ineach of the charging paths L1 and L2. In the charge circuit 460A, thenumber of series diodes is “1”. In contrast, in the charge circuit 460B,the number of series diodes is “3”. If the forward voltage of the diodeD is 0.6 V, the voltage drop amount of the charge circuit 460A is 0.6 V,whereas the voltage drop amount of the charge circuit 460B is 0.6×3. Thebattery 400 has the merit of being capable of changing the amount ofdrop in charge voltage by switching between the charge circuits 460A and460B. Further, the charge circuit 460B can use a voltage drop elementother than a diode. By setting the voltage drop amount of the chargecircuit 460B to a value assuming a case in which a 24-V system or 48-Vsystem charger is connected, the 12-V system battery 400 can be chargedeven with a charger of a voltage class other than 12-V system, such as24-V system or 48-V system.

(13) In the first embodiment, when the total voltage Ev of the assembledbattery 30 is higher than the charge set voltage Eo, as shown in FIG. 5, the FET 63A in the first charging path 61A and the FET 63B in thesecond charging path 61B are controlled to be turned off, whereas theFET 65A in the first charging path 61A and the FET 65B in the secondcharging path 61B are controlled so as to be alternately switched on andoff every half cycle Ts/2. In addition to this, the ON times of the twoFETs 65A and 65B may be changed according to temperature conditions forthe charging paths 61A and 61B. FIG. 17 shows how each FET is turned onand off when a temperature Ta of the FET 63A is higher than atemperature Tb of the FET 63B (Ta>Tb). The CPU 101 sets an ON time Ton1of the FET 65A in the first charging path 61A having a high temperatureto be shorter than an ON time Ton2 of the FET 65B in the second chargingpath 61B having a low temperature.

The on-times Ton1 and Ton2 of the FETs 65A and 65B are changed accordingto the temperature Ta of the FET 63A and the temperature Tb of the FET63B. When the temperature Ta of the FET 63A is lower than thetemperature Tb of the FET 63B (Ta<Tb), the ON time Ton2 of the FET 65Bin the second charging path 61B having a high temperature is set shorterthan the ON time Ton1 of the FET 65A in the first charging path 61Ahaving a low temperature. This makes the time during which the chargingpath with higher temperature is deenergized longer than the time duringwhich the charging path 61B with lower temperature is deenergized,thereby suppressing heat generation in the charging path with highertemperature and reducing the temperature difference from the chargingpath with lower temperature.

(14) The second embodiment has exemplified the method of switchingbetween the charging paths according to the temperature differencebetween the FETs 63A and 63B. Alternatively, the charging paths may beswitched when one of the two FETs 63A and 63B reaches a thresholdtemperature.

(15) In the first embodiment, when the total voltage Ev of the assembledbattery 30 exceeds the charge set voltage Eo, the CPU 101 alternatelyturns on the FET 65A in the first charging path 61A and the FET 65B inthe second charging path 61B to make charge currents alternately flowthrough the two charging paths 61A and 61B. As shown in FIG. 18 , duringthe switching control of the charging paths 61A and 61B, the CPU 101 maycontrol the two FETs 65A and 65B so as to ensure an overlapping period Win which both the FET 65A in the first charging path 61A and the FET 65Bin the second charging path 61B are tuned on. Ensuring the overlappingperiod W makes it possible to prevent the two FETs 65A and 65B frombeing turned off at the same time and the charging from being stoppedwhen the charging paths 61A and 61B are switched. In order to providethe overlapping period W, the ratio of an ON time Ton of the FETs 65Aand 65B to the control cycle Ts may be set to be higher than 50%. Forexample, when the ratio of the on time Ton to the control cycle Ts is60%, 10% of the control cycle Ts can be set as the overlapping period W.

(12) The techniques disclosed in the first and second embodiments can beimplemented in various forms such as a charging path switching programfor charging an energy storage device and a recording medium recordingthe program.

A switching program is a charging path switching program for chargingenergy storage devices. This program causes a computer (management unit100) to, in a configuration in which the plurality of charging paths areconnected in parallel and each charging path is provided with a voltagedrop element and a switch connected in series, perform processing (S40)of controlling the switches to switch a charging path, of the pluralityof charging paths, which is deenergized during charging.

REFERENCE SIGN LISTING

-   -   20: battery (corresponding to “energy storage apparatus”        according to present invention)    -   30: assembled battery    -   41: current sensor    -   45: voltage detector    -   50: charge control apparatus    -   60: charge circuit    -   61A: first charging path    -   61B: second charging path    -   63A, 65A: FET    -   64A, 66A: parasitic diode    -   63B, 65B: FET    -   64B, 66B: parasitic diode    -   100: management unit    -   101: CPU (corresponding to “control unit” according to present        invention)

The invention claimed is:
 1. A charge control apparatus that controlscharging of an energy storage device, the charge control apparatuscomprising: a plurality of charging paths connected in parallel for theenergy storage device; and two back-to-back connected field effecttransistors (FETs) with built-in parasitic diodes and a switch connectedin series on each of the plurality of charging paths; and a controlunit, wherein: the control unit controls each of the switches to switcha particular charging path, of the plurality of charging paths, which isdeenergized during charging; the controlled switching of the particularcharging path is based on a temperature difference calculated based upona difference between detected temperatures across the respective voltagedrop elements provided in each of the plurality of charging paths; thecontrol unit performs control to simultaneously turn on the twoback-to-back connected FETs in at least one of the charging pathsconnected in parallel when a charge voltage or a voltage of the energystorage device is lower than a set voltage; and the control unitperforms control, when a charge voltage or a voltage of the energystorage device is higher than a set voltage, to: turn off the FETs, ofthe two FETs installed in each of the charging paths and back-to-backconnected, in which the built-in parasitic diodes are in a forwarddirection with respect to a charging direction, and turn on the FETs, ofthe two FETs installed in each of the charging paths and back-to-backconnected, in which the built-in parasitic diodes are in a reversedirection with respect to the charging direction at different timingsbetween the charging paths.
 2. The charge control apparatus according toclaim 1, wherein the control unit further executes switching between thecharging paths when a charge voltage or a voltage of the energy storagedevice is higher than a set voltage.
 3. The charge control apparatusaccording to claim 1, wherein the control unit further switches betweenthe charging paths based on a set time.
 4. The charge control apparatusaccording to claim 1, wherein the control unit further switches betweenthe charging paths such that a time during which the charging path withhigher temperature is deenergized is longer than a time during which thecharging path with lower temperature is deenergized.
 5. The chargecontrol apparatus according to claim 1, wherein the control unit furthercontrols the switches in the charging paths such that during theswitching control of the charging paths, the switches in the chargingpaths have an overlapping period in which the switches aresimultaneously turned on.
 6. An energy storage apparatus comprising: anenergy storage device; and a charge control apparatus according toclaim
 1. 7. A method for charging an energy storage device using aplurality of charging paths connected in parallel, the charging pathseach including a voltage drop element and a switch connected in series,the method comprising controlling the switches to switch a particularcharging path, of the plurality of charging paths, which is deenergizedduring charging, while performing charging upon lowering a chargevoltage by using the voltage drop elements, the controlled switching ofthe particular charging path being based on a temperature differencecalculated based upon a difference between detected temperatures acrossthe respective voltage drop elements provided in each of the pluralityof charging paths.
 8. A charge control apparatus that controls chargingof an energy storage device, the charge control apparatus comprising: aplurality of charging paths connected in parallel for the energy storagedevice; and two back-to-back connected field effect transistors (FETs)with built-in parasitic diodes a switch connected in series on each ofthe plurality of charging paths; and a control unit, wherein: thecontrol unit controls each of the switches to switch a particularcharging path, of the plurality of charging paths, which is deenergizedduring charging; the control unit, when a charge voltage or a voltage ofthe energy storage device is higher than a set voltage, executes asequential switching between the plurality of charging paths in ashifted manner at predetermined time intervals so as to maintain currentflow from multiple of the plurality of charging paths; the control unitperforms control to simultaneously turn on the two back-to-backconnected FETs in at least one of the charging paths connected inparallel when a charge voltage or a voltage of the energy storage deviceis lower than a set voltage; and the control unit performs control, whena charge voltage or a voltage of the energy storage device is higherthan a set voltage, to: turn off the FETs, of the two FETs installed ineach of the charging paths and back-to-back connected, in which thebuilt-in parasitic diodes are in a forward direction with respect to acharging direction, and turn on the FETs, of the two FETs installed ineach of the charging paths and back-to-back connected, in which thebuilt-in parasitic diodes are in a reverse direction with respect to thecharging direction at different timings between the charging paths. 9.The charge control apparatus according to claim 8, wherein the controlunit executes switching between the charging paths when a charge voltageor a voltage of the energy storage device is higher than a set voltage.10. The charge control apparatus according to claim 8, wherein thecontrol unit switches between the charging paths based on a set time.11. The charge control apparatus according to claim 8, wherein thecontrol unit switches between the charging paths such that a time duringwhich the charging path with higher temperature is deenergized is longerthan a time during which the charging path with lower temperature isdeenergized.
 12. The charge control apparatus according to claim 8,wherein the control unit controls the switches in the charging pathssuch that during the switching control of the charging paths, theswitches in the charging paths have an overlapping period in which theswitches are simultaneously turned on.
 13. An energy storage apparatuscomprising: an energy storage device; and a charge control apparatusaccording to claim 8.